Polyunsaturated fats are lipids in which the constituent hydrocarbon chain possesses two or more carbon–carbon double bonds.[1][2] Polyunsaturated fat can be found mostly in nuts, seeds, fish, algae, leafy greens, and krill. "Unsaturated" refers to the fact that the molecules contain less than the maximum amount of hydrogen (if there were no double bonds). These materials exist as cis or transisomers depending on the geometry of the double bond.

Saturated fats have hydrocarbon chains which can be most readily aligned. The hydrocarbon chains in trans fats align more readily than those in cis fats, but less well than those in saturated fats. This means that, in general, the melting points of fats increase from cis to trans unsaturated and then to saturated. See the section on chemical structure of fats for more information.

The position of the carbon-carbon double bonds in carboxylic acid chains in fats is designated by Greek letters. The carbon atom closest to the carboxyl group is the alpha carbon, the next carbon is the beta carbon and so on. In fatty acids the carbon atom of the methyl group at the end of the hydrocarbon chain is called the omega carbon because omega is the last letter of the Greek alphabet. Omega-3 fatty acids have a double bond three carbons away from the methyl carbon, whereas omega-6 fatty acids have a double bond six carbons away from the methyl carbon. The illustration below shows the omega-6 fatty acid, linoleic acid.

Among n-3 fatty acids [Omega-3], neither long-chain nor short-chain forms were consistently associated with breast cancer risk. High levels of docosahexaenoic acid (DHA), however, the most abundant n-3 PUFA [Omega-3] in erythrocyte (red blood cell) membranes, were associated with a reduced risk of breast cancer.[5] The DHA obtained through the consumption of polyunsaturated fatty acids is positively associated with cognitive and behavioral performance.[6] In addition DHA is vital for the grey matter structure of the human brain, as well as retinal stimulation and neurotransmission.[7]

The importance of the ratio of omega-6/omega-3 essential fatty acids as established by comparative studies shows an Omega-6:Omega-3 ratio under 4:1 may contribute to health. [10]

Due to the lack of pre-formed EPA and DHA in the vegan diet, High doses of ALA are effective in providing limited amounts of EPA, and very poor amounts of DHA in Vegan Populations.[11]

There are conflicting associations between dietary factors and incident atrial fibrillation (AF). A 2010 study published in AJCN suggested that polyunsaturated fats were found to have no significant association with AF.[12]

Contrary to conventional advice, an evaluation of evidence from 1966-1973 pertaining to the health impacts of replacing dietary saturated fat with polyunsaturated fat found that participants in the group doing so had increased rates of death from all causes, coronary heart disease, and cardiovascular disease. Although this evaluation was criticized by many scientists, it fueled debate over worldwide dietary advice to substitute polyunsaturated fats for saturated fats.[13][14] Furthermore, polyunsaturated fats (along with partially hydrogenated fats, fried foods, and smoking) are implicated in a massive increase of sphingomyelin, a oxysterol which clogs arteries[15] and weakens muscle contraction.[16]

Consumption of omega-3 fatty acids during pregnancy is critical to fetal development. They are required during the prenatal period for the formation of synapses and cell membranes. These processes are also essential in postnatal human development for injury response of the central nervous system and retinal stimulation.[11]

A study published in The Journal of Nutrition in 2007 found that the maternal diet of rats affects the brain DHA status of offspring. It suggests that a maternal diet containing insufficient amounts of omega-3 polyunsaturated fatty acid can lead to greater risk of decreased accretion of brain DHA in the rats' offspring.[17]

A 2010 study of 3,081 women suffering from breast cancer was carried out to research the effects of polyunsaturated fats on breast cancer. It demonstrated that the consumption of high amounts of long chain omega-3 polyunsaturated fats from food produced a 25% reduced risk of additional breast cancer events. These women were also shown to have reduced risk of “all-cause mortality.” Consumption of polyunsaturated fats through fish oil supplements was not shown to decrease risk of recurring breast cancer events, although the authors noted that consumption of supplements among the group was low, less than 5%.[18]

At least one study in mice has indicated that consuming high amounts of polyunsaturated fat (but not monounsaturated fat) may increase metastasis of cancer in the rats.[19] The researchers found that linoleic acid in polyunsaturated fats produced increasing membrane phase separation, and thereby increased adherence of circulating tumor cells to blood vessel walls and remote organs. According to the report 'The new findings support earlier evidence from other research that consuming high amounts of polyunsaturated fat may increase the risk of cancer spreading'. The propensity for polyunsaturated fats to oxidize is another possible risk factor.[20][21] This leads to the generation of free radicals and eventually to rancidity. Studies have shown that low dosages of Coenzyme Q10 reduce this oxidation, and a combination of a diet rich in polyunsaturated fatty acids and Coenzyme Q10 supplementation leads to a longer lifespan in rats.[22] Studies on animals have shown a link between polyunsaturated fat and the incidence of tumours. In some of these studies the incidence of tumours increased with increasing intake of polyunsaturated fat, up to about 5% of total energy, near to the middle of the current dietary intake in humans. It is advised that the level of polyunsaturated fats in the diet be regulated if Coenzyme Q10 supplements are not being taken. However, even without Coenzyme Q10 supplementation, the effect on health might be considered by some to be more beneficial than harmful, due to the supposed cholesterol lowering effects of unsaturated fats compared to saturated fats;[citation needed] however, (a) monounsaturated fats have also been posited to lead to lower cholesterol levels; and (b) it is no longer clear that saturated fats actually cause elevated blood cholesterol levels.[23] See saturated fat and cardiovascular disease controversy.